Ensure that you have installed the Citation Plugin. To do so, in Mendeley Desktop, go to Tools > Install MS Word Plugin. The plugin is available for both MS Word for Windows and Mac.
Select the References tab in the Word toolbar. You will see the Mendeley Cite-O-Matic on the ribbon
Depending on your version of Word, you will either see the Mendeley Cite-O-Matic on the ribbon under Add in (Word 2016), or as a separate toolbar (earlier versions). If the tool bar is missing, go to View > Toolbars > Mendeley Toolbar.
For additional details on using the Citation Plugin from Mendeley, click here.
This video shows you how to insert citations and generate bibliographies with Mendeley:
1) Installing the Citation Plugin (0:15)
2) Displaying the Mendeley toolbar in Word (0:26)
3) Inserting citations using the toolbar (0:34)
4) Inserting citations from your Mendeley Library (1:00)
5) Searching and inserting citations from Groups (1:24)
6) Creating bibliographies (1:33)
7) Changing between citation styles (1:52)
Note: This video was released by Mendeley in 2011. The interface for Mendeley and/or Word in the video may look different from newer versions in use now.